Sintered composite body comprising cemented carbide and cbn grains

ABSTRACT

A sintered composite body of cemented carbide and cBN grains, wherein the cBN grains are dispersed in a cemented carbide matrix. The body further includes a cBN depleted zone extending 50-400 μm from the surface of the body towards the core thereof. The mean cBN grain size outside the depleted zone is 1-20 μm and the cBN content outside the depleted zone is 0.3-4 wt %.

TECHNICAL FIELD

The present invention relates to a sintered composite body comprisingcemented carbide and cBN grains, wherein the cBN grains are dispersed ina cemented carbide matrix and wherein the mean cBN grain size is 1-20 μmand the cBN content is 0.3-4 wt %.

BACKGROUND

Cemented carbide components are used in a wide range of applications,especially in components subjected to extreme wear under abrasiveconditions. In the oil, gas and mining industry cemented carbide is acommonly used material in several important components, from drillingbits to general wear parts. The most important features of suchcomponents are a combination of high surface hardness and hightoughness.

Cubic boron nitride (cBN) is a superhard material surpassed only bydiamond in hardness, which is widely used in demanding applications suchas machining tools. cBN is generally crystallographic stable attemperatures below 1400° C.

One way of increasing the wear resistance of cemented carbide in forexample cutting applications is to add particles of a hard material likecBN grains. This has been disclosed for example in EP 0256829 where ahigh pressure of 50-70 kilo bars is applied during the sintering.

A problem with a material with increased wear resistance is that alsothe wear resistance during grinding is increased. Grinding is a commonfinal treatment during production, aimed to achieve a desired shape andsurface finish of a product, for example a cutting tool or a saw tooth.Due to an improved wear resistance this step can be costly and alsodemanding for certain geometries.

SUMMARY

It is an object of the present invention to provide a cemented carbidebody comprising grains of cBN, which is easier to grind into its finalshape compared to prior art. It is a further object of the presentinvention to provide a cemented carbide body comprising grains of cBNwith a predetermined grinding property. It is a further object of thepresent invention to provide a cemented carbide body comprising grainsof cBN, with an improved joinability compared to prior art.

At least one of these objects is achieved by a sintered composite bodyaccording to claim 1. Preferred embodiments are disclosed in thedependent claims.

The present invention relates to a sintered composite body comprisingcemented carbide and cBN grains, wherein the cBN grains are dispersed ina cemented carbide matrix, wherein said body comprises a cBN depletedzone extending from the surface of the body and 50-400 μm, preferably100-300 μm, towards the core of the body and the mean cBN grain sizeoutside the depleted zone is 1-20 μm, preferably 1-10 μm, morepreferably 2-8 μm, and the cBN content outside the depleted zone is0.3-4 wt %, preferably 0.3-2 wt %, more preferably 0.5-1 wt %.

The cemented carbide matrix comprises hard constituents in a metallicbinder phase. The binder phase can comprise one or more selected fromthe group of Co, Ni and Fe and the hard constituents can comprise WC.The cemented carbide can further comprise hard constituents selectedfrom borides, carbides, nitrides or carbonitrides of metals from groups4, 5 or 6 of the periodic table, preferably tungsten, titanium,tantalum, niobium, chromium and/or vanadium.

The manufacturing of the sintered composite body typically comprisesmixing and milling powders of the cemented carbide and cBN, pressingbodies of the powder to a desired shape and finally sintering thepressed bodies to form dense bodies comprising cBN grains in a cementedcarbide matrix. During the sintering process the binder phase liquidizeand enclose the hard constituents and the cBN grains. The depleted zoneforms during the sintering step, which is disclosed in more detailbelow. The milling, mixing and pressing steps can be performed withconventional methods as known in the art.

The cBN grain size does typically not change during the mixing, millingand pressing steps. The surface of the cBN grain can be coated with athin layer of a metal element, for example a thin Ti coating, toincrease the wetting performance of the grain surface during thesintering step.

The body of the present invention can be of any shape, for example inthe shape of a saw tooth, a drilling button or a wire drawing nib. Thecore in the body is located inside the body. The shape and extension ofthe core depends on the shape of the body. For example, in a sphericalbody the core can be a central point, in a body extended in onedirection, the core can be extended, and in a ring shaped body, the corecan be ring shaped or cylindrically shaped.

By “cBN depleted zone” is hereby meant an area that in SEM analysis at750× magnification does show that the cBN grains, that normally appearas black spots or small areas in an otherwise continuous matrix ofcemented carbide, are missing or depleted. The depleted zone isessentially free of cBN grains and it extends from the surface of thebody and down below the surface towards the core of the body. The areawhere the cBN grains are not depleted extends outside the depleted zonefor example towards and through the core of the body.

One advantage with the sintered composite body according to the presentinvention is that the grinding of the surface of the body, i.e. grindingof the depleted zone, is more easy to perform due to the fact that thehard particles of cBN are missing in the outer surface area of the body.The depleted zone is less wear resistant than the cBN containingmaterial in the core of the body.

Another advantage with the sintered composite body according to thepresent invention is that joining of the body to another body ormaterial can be improved. At brazing or welding the strength of theweld, i.e. the melt region, is dependent on the strength of the materialin the weld. cBN grains are not preferred in a weld due to the fact thatcBN grains have a thermal mismatch with the cemented carbide matrix. Anadvantage with an absence of cBN grains in the weld is that it leads toan absence of stresses due to said thermal mismatch. Also, thewettability of materials in the weld could be improved if no cBN ispresent. Additionally, cBN as a brittle phase is not present in thejoint area. All of these facts lead to an improved joining strength ifno cBN is present in the joint. In general, absence of cBN in thewelding zone makes it possible to use existing production processparameters for welding and plating, thus reducing the production costs.

In one embodiment of the present invention, the extension of thedepleted zone is 50-200 μm, preferably 100-200 μm. This is preferred inapplications focusing on achieving a good surface finish and/or a smallradius after surface or cutting edge grinding operations.

In one embodiment of the present invention, the extension of thedepleted zone is 200-400 μm, preferably 200-300 μm. This is preferred inapplications requiring high toughness to withstand initial impact. Ifsuch a body withstands the initial impact, it thereby has an increasedchance to wear with a stable wear rate.

In one embodiment of the present invention, the cemented carbidecomprises 6-16 wt % binder phase. In one embodiment of the presentinvention the binder phase comprises Co. In one embodiment of thepresent invention the cemented carbide comprises 10-14 wt % Co.

In one embodiment of the present invention, the cemented carbidecomprises WC. In one embodiment of the present invention, the cementedcarbide comprises 80-94 wt % WC. In one embodiment of the presentinvention the mean WC grain size is 0.5 to 8 μm, preferably 0.5 to 4 μm,most preferably 0.8 to 1.2 μm, as measured with linear intercept methodin the sintered material.

In one embodiment, the present invention relates to a wear partcomprising the sintered composite body according to the above.

In one embodiment, the present invention relates to a saw toothcomprising the sintered composite body according to the above.

The present invention further relates to the use of the sinteredcomposite body in oil or gas applications, for example as a drillingbutton or an insert for a drilling head.

The present invention further relates to the use of the sinteredcomposite body in wire drawing applications, for example as a wiredrawing nib.

The composite body according to the present invention can be sintered ina sintering process in accordance with the settings as indicated below.

The sintering temperature is preferably 1250-1360° C., preferably1300-1360° C. At a too low sintering temperature, the material will notsinter. It is important to reach the melting point of the binder. A toohigh sintering temperature results in that the cBN grains decompose intohBN, which is a less hard phase of BN. The sintering temperature ispreferably chosen to achieve fully densified bodies and a gradient zoneof a preferable depth.

The sintering can be performed in vacuum. Vacuum sintering is a standardprocess of production for many cemented carbide manufacturers.

The sintering can be performed using HIP (hot isostatic pressing).Sintering using a HIP is advantageous in that it leads to higherdensities of the materials. It also enables a lower sinteringtemperature compared to what is possible at vacuum sintering,maintaining full densification of the material.

The sintering can for example be performed in a gas comprising Ar and/orN₂.

The temperature is held at a sintering temperature during a dwellingtime of preferably 10-80 minutes. A too long sintering time can resultin undesired grain growth of the cemented carbide. A too short sinteringtime can result in not completely sintered material at the centre of abody. The sintering time is suitably adjusted with regards to batchsize, sintering equipment, cemented carbide composition, size of bodies,etc. to achieve dense sintered bodies with a preferable gradient depth.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings andclaims.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, wherein:

FIG. 1 is a SEM picture of a polished through cut of a sinteredcomposite body according to a preferred embodiment of the presentinvention, wherein the grain size of the cBN is 4-8 μm (Sample C). Theepoxy resin 1, the depleted zone 2 and an area in the material that isnot depleted 3 are indicated. The cBN grains 5 appear as dark spots. Thewidth of the depleted zone is indicated with an arrow 4.

FIG. 2 is a SEM picture corresponding to FIG. 1, but wherein the grainsize of the cBN is 2-4 μm (Sample A).

DETAILED DESCRIPTION

In the following, examples of bodies according to different embodimentsof the invention will be presented, and the method of making the bodieswill be disclosed in detail.

cBN powder comprising cBN grains was milled for 1 hour in a 250 mllaboratory ball mill. After milling of the cBN, cemented carbide powdercomprising 86.98 wt % WC, 0.62 wt % Cr₃C₂ and 12.4 wt % Co was added.The amount of cBN was adjusted to equal 0.78 wt % cBN in the mixturewith cemented carbide. After the addition of cemented carbide powder,the mixture was milled for another 30 minutes. During the whole process,a mixture of ethanol and water was used as a milling liquid. Aftermilling, the slurry was poured onto a tray and dried over night at 70°C. After that, the powder was sieved using a 500 micron mesh.

Two cBN grain sizes were analyzed: 2-4 μm and 4-8 μm respectively. ThecBN grains were, as delivered from the manufacturer, coated with a thincoating of Ti. The given size of the cBN grains is the size specified bythe manufacturer.

The dry powder was pressed to a body of a bar with rectangularcross-section. As-pressed dimensions were about 25.5×8×6.5 mm.

The bodies were sintered in sintering steps defined below. The followingsintering parameters were analyzed: temperature, time, pressure andsintering gas, see Tables 2-5 below.

After the sintering each body were cut through and the through cut wasstudied in SEM at 750× magnification whereby the depth of the depletedzone was studied. Examples of SEM pictures of the depleted zone areshown in FIGS. 1 and 2.

TABLE 1 (grain size) cBN grain Pressure Temperature Time Depleted Samplesize (μm) (mbar) Gas (° C.) (min) zone (μm) A 2-4 5 Ar 1360 70 220 B 4-85 Ar 1360 70 180

As can be seen in Table 1, the extension of the depleted zone isdependent on the grain size of the cBN particles. After a sintering atequal conditions, the sample A with smaller grains of 2-4 μm had alarger width (depth) of the depleted zone compared to the sample B withthe larger grains of 4-8 μm.

TABLE 2 (sintering temp) cBN grain Pressure Temperature Time DepletedSample size (μm) (mbar) Gas (° C.) (min) zone (μm) C 4-8 5 Ar 1300 70110 B 4-8 5 Ar 1360 70 180

As can be seen in Table 2, the extension of the depleted zone isdependent on the sintering temperature. After a sintering at equalconditions but at different temperatures, the sample C sintered at 1300°C. had a smaller width (depth) of the depleted zone compared to thesample B sintered at 1360° C.

TABLE 3 (sintering time) cBN grain Pressure Temperature Time DepletedSample size (μm) (bar) Gas (° C.) (min) zone (μm) D 2-4 50 Ar 1360 70280 E 2-4 50 Ar 1360 35 240

As can be seen in Table 3, the extension of the depleted zone is alsodependent on the sintering time. After a sintering at equal conditionsbut at different sintering times, the sample D sintered at 70 minuteshad a larger width (depth) of the depleted zone compared to the sample Esintered at 35 minutes.

TABLE 4 (sintering pressure) cBN grain Pressure Temperature TimeDepleted Sample size (μm) (bar) Gas (° C.) (min) zone (μm) A 2-4 0.005Ar 1360 70 220 D 2-4 50 Ar 1360 70 280 F 2-4 1 N₂ 1360 70 165 G 2-4 50N₂ 1360 70 275

As can be seen in Table 4, the extension of the depleted zone is alsodependent on the sintering pressure. After a sintering at equalconditions in Ar gas, but at different pressures, the sample A sinteredat 5 mbar had a smaller width (depth) of the depleted zone compared tothe sample D sintered at 50 bar. The same relation is valid forsintering in N₂ gas: the sample F sintered at atmospheric pressure had asmaller width (depth) of the depleted zone compared to the sample Gsintered at 50 bar.

TABLE 5 (sintering gas) cBN grain Pressure Temperature Time DepletedSample size (μm) (bar) Gas (° C.) (min) zone (μm) D 2-4 50 Ar 1360 70280 G 2-4 50 N₂ 1360 70 275

As can be seen in Table 5, the sintering gas Ar or N₂ did not have anyclear diverging effect on the width (depth) of the depleted zone. At thesintering conditions shown in Table 5, the depleted zone for sample Dand G had about the same width (depth) of the depleted zone.

While the invention has been described in connection with variousexemplary embodiments, it is to be understood that the invention is notto be limited to the disclosed exemplary embodiments, on the contrary,it is intended to cover various modifications and equivalentarrangements within the appended claims.

1. A sintered composite body comprising cemented carbide and cBN grains,wherein the cBN grains are dispersed in a cemented carbide matrix,wherein said body comprises a cBN depleted zone having a gradient depthof 50-400 μm extending from a surface of the body towards a core of thebody and a mean cBN grain size outside the depleted zone is 1-20 μm anda cBN content outside the depleted zone is 0.3-4 wt %, the depleted zonebeing formed by a method comprising the steps of sintering the body at atemperature of 1250-1360° C., in a gas selected from the group of Ar orN₂ and holding the sintering temperature for a dwelling time of 10-80minutes.
 2. The sintered composite body according to claim 1, whereinthe cBN depleted zone gradient depth is 50-200 μm.
 3. The sinteredcomposite body according to claim 1, wherein the cBN depleted zonegradient depth is 200-400 μm.
 4. The sintered composite body accordingto claim 1, wherein the cemented carbide comprises 6-16 wt % binderphase.
 5. The sintered composite body according to claim 4, wherein thebinder phase comprises Co.
 6. The sintered composite body according toclaim 4, wherein the cemented carbide comprises 10-14 wt % Co.
 7. Thesintered composite body according to claim 1, wherein the cementedcarbide comprises WC.
 8. The sintered composite body according to claim7, wherein the cemented carbide comprises 80-94 wt % WC.
 9. The sinteredcomposite body according to claim 7, wherein the mean WC grain size is0.5-8 μm.
 10. A wear part of a sintered composite body comprisingcemented carbide and cBN grains, wherein the cBN grains are dispersed ina cemented carbide matrix, wherein said body comprises a cBN depletedzone having a gradient depth of 50-400 μm extending from a surface ofthe body towards a core of the body and a mean cBN grain size outsidethe depleted zone is 1-20 μm and a cBN content outside the depleted zoneis 0.3-4 wt %, the depleted zone being formed by a method comprising thesteps of sintering the body at a temperature of 1250-1360° C., in a gasselected from the group of Ar or N₂ and holding the sinteringtemperature for a dwelling time of 10-80 minutes.
 11. A saw tooth of asintered composite body comprising cemented carbide and cBN grains,wherein the cBN grains are dispersed in a cemented carbide matrix,wherein said body comprises a cBN depleted zone having a gradient depthof 50-400 μm extending from a surface of the body towards a core of thebody and a mean cBN grain size outside the depleted zone is 1-20 μm anda cBN content outside the depleted zone is 0.3-4 wt %, the depleted zonebeing formed by a method comprising the steps of sintering the body at atemperature of 1250-1360° C., in a gas selected from the group of Ar orN₂ and holding the sintering temperature for a dwelling time of 10-80minutes.
 12. (canceled)
 13. (canceled)
 14. The sintered composite bodyof claim 1, wherein the sintering temperature is 1300-1360° C.